The invention is directed to a shaped catalyst, particularly for the formation and/or equipping of reaction chambers with catalytically active surfaces, a process for the production of these catalysts, especially in directly insertable spatial conformations as well as their use for purifying waste gas.
The noxious materials of waste gas, particularly exhaust gases of internal combustion motors of the continuously increasing number of motor vehicles, presents a considerable danger to the health of men, animals and the plant world. In several countries, they are presently limited through laws regulating the maximal permissible toxic material concentrations. Among the already proposed and practiced methods for the solution of these air pollution problems the catalytic waste gas purification processes are the most important. In regard to behavior in heating, effectiveness, duration of activity and mechanical stability the highest demands are placed on the needed catalysts. For example, when inserted in motor vehicles they must be effective at the lowest possible temperatures and guarantee for a long time in all important temperature and space velocity ranges a high percentage reaction of the noxious materials to be removed (particularly hydrocarbons, carbon monoxide and nitric oxide) to the non-toxic oxidation and reduction products carbon dioxide, steam and nitrogen. Because of the severe mechanical load during the continued operation they must have sufficient mechanical stability and must not lose this even with long overheating as can occur in a given case through impact with unburned motor fuel caused by, for example, ignition failure in one or more cylinders. Thus they must satisfy a number of conditions which simultaneously are difficult to fulfill or are contrary to each other.
Previously besides particulate bed catalysts, i.e., pellets or extrudates of supported catalysts or interspersant or mixed catalysts, there have been primarily used monolithic supported catalysts. They consist of a catalytically inert, low surface area ceramic skeleton of, e.g., cordierite, mullite or .alpha.-aluminum oxide as structural reinforcing support on which there is placed a thin, for the most part high surface area, layer of a heat resistant, for the most part oxidic carrier material such as aluminum oxide of the so-called gamma form, which latter in turn supports the true catalytically active components.
These can consist of noble metal, noble metal compounds or non-noble metal compounds or mixtures thereof. From the group of noble metals there are added, for example, platinum, palladium, rhodium, ruthenium, iridium, gold and silver. As non-noble metal compounds there can be used, for example, the oxides of copper, chromium, manganese, iron, cobalt, nickel and their combinations, as, for example, copper chromite. Further variants arise by combining noble metals or their compounds with non-noble metals or their compounds or non-noble metals or their compounds with noble metals or their compounds. In many cases there are added to the active components slight amounts of other elements, for example, from the group of alkaline earth metals such as magnesium, calcium, strontium or barium, from the group of rare earths such as, e.g., samarium, lanthanum, cerium or from the fourth group of the periodic system, as, e.g., titanium, zirconium or tin, as so-called promoters for improving specific properties of the system. Intermetallic compounds and alloys of the above metals and compounds may also be used.
An important disadvantage of catalysts made using ceramic structural reinforcing supports, particularly of monolithic honeycomb catalysts of cordierite, mullite or .alpha.-aluminum has proven to be their sensitivity toward mechanical influences and thermal overheating. Thus the vibrations occurring in operation of the unit through the intermittent impulses of the exhaust gas column, the motor vibrations and travelling motion in combination with temperature peaks exert wear and fracture causing forces on the ceramic. With thermal overheating taking place within the narrow spatial confines of the monoliths there can occur sintering, melting and baking of the structurally reinforcing agents with their coatings present in the form of monoliths or pellets, from which partial or complete inactivation results.
Furthermore, it has been shown that the installation of such ceramic honeycombs in metal housings is difficult because of the different heat expansion characteristics of ceramic and metal and requires expensive construction precautions in order to guarantee with the relative motion occurring elastic and gas tight support of the honeycomb continuously in the highest possible interval in the temperature range between -30.degree. and +1000.degree. C.
There has not been a lack of endeavors to replace the ceramic based materials by better suited synthetic materials and to attempt to seek a favorable spatial design for this.
Thus there has already been described in German O.S. No. 2,302,746 a carrier matrix for a catalytic reactor for waste gas purification of internal combustion engines which is manufactured from a variably arranged corrugated and smooth high temperature resistant steel sheet, which is coated with a catalytically acting metal, such a platinum or palladium, or a metal oxide, such as copper oxide, nickel oxide or the like. There is also described, to insert metal carriers of materials having a high nickel content (Monel metal) whereby the nickel after conversion into the oxide show catalytic activity. The foregoing Offenlegungsschrift (published patent application) among others proposes either to coat the steel sheets with copper or nickel and to subsequently oxidize the coating or to directly coat it with a catalytically active metal oxide.
The German published patent applications Nos. 2,436,559 and 2,540,882 suggest both a simplified catalytic reactor for detoxification of the exhaust gases of an internal combustion engine in which the exhaust pipe and/or the manifold line and/or the discharge pipe optionally provided with gas agitating devices are coated internally with a catalytically active layer of platinum, copper or are wash coated with a gamma aluminum oxide which itself is coated with platinum.
All of the previously mentioned solutions, of course, produce a usable constructive solution for the spatial development of structural reinforcing agents to which the structural parts of the exhaust gas lines also belong. However, they do not show any way for the production of a sufficiently adhesive union between metal and catalyst carrier.
The first attempt for a treatment of this decisive problem for catalyst stability is found in German O.S. No. 2,450,664. There is described a catalyst constructed of maximally four layers, in which a heat and oxidation resistant carrier of an iron alloy in the form of an expanded sheet wound into a roll is covered with an oxygen containing, porous coating which should receive the catalyst layer arranged thereon and preferably is preserved by the thermal, chemical or electrolytical surface oxidation of an aluminum containing iron alloy with eventual additional reinforcement by means of an externally applied aluminum oxide (a likewise described alternative procedure consisting in the direct coating of the metal with aluminum oxide from dispersions is irrelevant to the problem addressed and because of the previously mentioned reasons is also unusuable).
The disadvantages of such type of oxide-coated catalysts are seen in that they require expensive heat and oxidation resistant iron alloys for the structural reinforcing agent and the oxide layer only is sufficiently adhesive to the alloy, for example, against a wash coat to be applied, if the metal present in the iron alloy for oxide formation is contained in a concentration which is not too small and can be oxidized out of this to a sufficient extent and with formulation of a porous, mechanically stable surface texture in a sufficiently economically time. As a further drawback of the known catalyst there must be mentioned the fact that its metallic structural reinforcer because of the additional oxide layer attached to it lead to reduced values for the heat transfer from the catalyst material impacted with exhaust gas which can lead to temporary overheating of the catalyst material.
A satisfactory solution for the adhesive anchoring of catalysts to temperature resistant metallic supports consists in the flame spraying method and is described in German Pat. No. 2,151,416. It is also already proposed there to coat any shaped construction surface in the reaction chamber for catalytic reaction of gases with catalyst material. However, the disadvantage is that only special oxidation catalysts, the so-called mixed catalysts, or interspersant type catalysts are usable and an additional application of copper or silver metal is necessary as an adhesive primer.